Treatment of carbon or graphite fibers and yarns for use in fiber reinforced composites

ABSTRACT

Carbon fiber or yarn as used in fiber reinforced composites is electrolytically treated to improve the surface characteristics and thereby to improve its bonding or adhesion to the matrix material. By this improved bonding, shear strengths of resultant fiber resin or plastic composites have been more than doubled. The electrolytic treatment is conducted by using the fiber or yarn as the anode and using en electrolyte such as an aqueous caustic solution.

United States Patent Ray et al. [451 June 20, 1972 1 TRE OF CARBON 0R2,807,577 9/1957 Antonsen ..204/130 GRAPHITE FIBERS AND YARNS FOR USE INFIBER REINFORCED Primary ExaminerJohn H. Mack COMPOSITES AssistantExaminerRv L. Andrews [72] Inventors: James D. Ray, Springfield; SamuelStein- Attorney-Harry Herbertvjn and Cedric Kuhn giser; Robert A. Cass,both of Dayton, all of Ohio Assignee: The United States of America asrepresented by the Secretary of the Air Force Filed: March 3, 1970 Appl.No: 16,251

U.S. Cl ..204/130 int. Cl. ll0lk 1/00 Field of Search ..204/130, 294

References Cited UNITED STATES PATENTS 6/1967 Olstowski ..204/130 5ABSTRACT Carbon fiber or yarn as used in fiber reinforced composites iselectrolytically treated to improve the surface characteristics andthereby to improve its bonding or adhesion to the matrix material. Bythis improved bonding, shear strengths of resultant fiber resin orplastic composites have been more than doubled. The electrolytictreatment is conducted by using the fiber or yarn as the anode and usingen electrolyte such as an aqueous caustic solution.

5 Claims, No Drawings TREATMENT OF CARBON OR GRAPHITE FIBERS AND YARNSFOR USE IN FIBER REINFORCED COMPOSITES BACKGROUND OF THE INVENTION 1.Field of the Invention.

This invention relates to carbon or graphite fiber or yarn reinforcingmaterials. In one aspect it relates to the treatment of the reinforcingfibers or yarns to improve the adhesion between the fiber surface and amatrix material.

2. Description of the Prior Art.

There has been an increasing demand for materials of construction ofhigh strength to weight ratio and high modulus to weight ratio for usein aerospace vehicles and devices, particularly for materials which alsohave good thermal stability and good shear strength.

Specifically, there has been a demand for improved reinforcing fibers tobe embodied in structural composites which form the components of theleading edges of high speed aircraft, the nose cones or heat shields forreentry vehicles, rocket engine components, and the like.

High strength and high modulus carbon fibers and yarns, particularlythose composed essentially of graphite and generally referred to asgraphite fibers and yarns, have already been used as reinforcing agentsin composite materials for aerospace structures. However, the bonding ofresins or plastics to the carbon fibers in such composites has not beenentirely satisfactory, and, as a result, the shear strengths of theresultant composites have been less than desired.

Attempts have been made to improve the surface adhesion properties byheating the fibers in an oxidizing atmosphere. However, this method hasbeen found to produce pitting and uneven results with over-oxidation andresultant weakening of the fibers in the easily accessible orpreferential areas and insufficient oxidation and poor adhesion in otherareas.

SUMMARY OF THE INVENTION The present invention resides in a processwhereby the bonding properties of carbon and graphite fibers or yarnsare improved to a surprising extent, in the improved fibers per se, andin the compositions incorporating the fibers. Broadly, the processcomprises the step of subjecting a carbon or graphite fiber to anelectrolytic reaction in an aqueous electrolyte whereby negative ionsare attracted to the surface of the fiber acting as an anode, therebymodifying the fiber surface. As a result of this modification,subsequent bonding to plastics and resins is improved to such an extentthat the shear strengths are increased in many cases to more than doublethe values obtained without this particular pretreatment with little orno loss in tensile strength.

During the electrolytic reaction, nascent oxygen forms at the anode andoxidizes the fiber surface. The improvement obtained by the presentprocess results from the uniformity of the fiber surface oxidation andthe accuracy with which the degree of oxidation can be controlled. Inaddition to the carbon to oxygen bonds that are formed at the fibersurface, it appears that hydroxyl groups and carboxyl groups also becomeattached to the surface and contribute to a marked extent to theimprovement in bonding properties.

In general, any electrolyte which will generate nascent oxygen at theanode can be used in the practice of this invention. Examples ofsuitable electrolytes include aqueous solutions of sodium hydroxide,potassium hydroxide, phosphoric acid, nitric acid, sulfuric acid, andthe like. It is usually preferred to employ aqueous sodium hydroxide oraqueous phosphoric acid solutions. It is to be understood that theconcentration of the solutions will have an efiect on the rate ofgeneration of nascent oxygen. For a practical rate of generation, it hasbeen found to be advantageous to use as the electrolyte a solutionhaving a concentration of 0.5 to 20, preferably 1 to ID, weight percent.

In effecting the electrolytic treatment, the carbon fiber or yarn isused as the anode as stated, and the cathode can be an electrode ofgraphite or other carbon form, or can be any metal or other materialsuitable for cathode purposes. The process can be applied either as abatch or continuous operation. In the latter case, a continuous carbonfiber or yarn is passed over or under rolls to guide the fiber throughthe solution, and connection with the current source is applied to thefiber either through one of the rolls in contact with the fiber, or byany other convenient means.

The length, thickness or other dimensions of the fibers are not criticaland are determined by other considerations in the ultimate purpose forwhich the product is to be used.

The temperature used is not critical except as it may affect the rate ofnascent oxygen reaction with the fiber surface. However, in such cases,slower rates of reaction can be compensated for by prolonged treatment.Therefore, whatever temperature is convenient, such as 2050C., is foundto be most practical.

In accordance with the broad concept of this invention, any currentdensity can be employed which is sufficient to produce nascent oxygen atthe anode in an amount great enough to modify the surface of the fiberso as to improve the bonding properties of the fiber. In general, acurrent density in the approximate range of 0.0005 to 0.005 ampere persquare centimeter (amp/cm") of the surface area of the yarn or fiber tobe treated has been found to be satisfactory. It is usually preferred toemploy a current density in the range of about 0.001 to 0.003 amp/cm?Any convenient voltage can be used in the electrolysis that will givethe desired current density. It is generally preferred to utilize avoltage between about 10 and volts. The period of the electrolyticreaction, referred to in the tables hereinafter as dwell or residencetime, usually ranges from about 25 to 500 seconds, preferably from about75 to 250 seconds. As previously mentioned, the temperature of theelectrolyte can conveniently range from about 20 to 50C.

The improvement in adhesion properties or bonding properties of thefibers of this invention occurs with any type of resin with which thefiber is mixed. Improvements in shear strengths occur for carbon fibercomposites in which the resin components include phenolic resins, epoxyresins, polyester resins, polyimide resins, polyamide resins as well aselastomers such as natural rubber, polybutadienes, polyisoprenes,butadiene-styrene copolymers, polyurethanes, and plastics such aspolyethylene, polypropylene, and the like. The composite generallycontains in the range of about 25 to 75 volume percent of the treatedcarbon fibers.

However, since the high strength high modulus carbon fibers arerelatively expensive and used only for particular purposes, such as inaerospace vehicles and devices, it is most practical to use resins whichare also suitable for such purposes. For that reason epoxy resins andother types of resins, such as phenolic resins, that are particularlysuitable for use in aerospace applications, are the ones preferred inmaking composites with the improved fibers of this invention.

Various methods of preparing carbon and graphite fibers are described inthe literature and various commercial products of this type areavailable.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The practice of this inventionis illustrated by the following examples. These examples are givenmerely for purpose of illustration and are not intended to be undulylimitative of the invention. Unless specifically indicated otherwise,parts and percentages are given by weight.

EXAMPLE I Several samples of commercial graphite yarn were taken fromthe same bobbin so as to have yarn as identical as possible for use inconducting comparative tests. The graphite yarn used in this andsucceeding examples was composed of two plies, each ply consisting of720 filaments. The yarn denier per ply was 351 (g/9,000 m). The filamentsurface area was 1 square meter per gram.

The electrolytic tests were conducted in a bath containing percentaqueous sodium hydroxide solution as the electrolyte and maintained atC. The cathode was a carbon electrode supported horizontally near thebottom of the tank and connected to a direct current source. Therespective samples of yarn were submerged in theelectrolyte by passingthem individually under two rollers supported completely submerged inthe bath. The positive pole of the direct current source was connectedto the continuous yarn being fed into the bath by means of a graphitecontact roll over which the yarn was passed prior to its entry into theelectrolyte bath. In each run the dwell or residence time of the yarn inthe bath was 150 seconds. The graphite yarn had a specific gravity of1.85 g./cc. After the electrolysis, each yarn was used as thereinforcement in a laminate prepared with an epoxy resin having aspecific gravity of 1.21 g./cc. The particular epoxy resin used was acommercially available product which was supplied as a twopackagesystem, one containing a mixture of bis-diglycidol ether of bisphenol Aand bis-2,3-epoxycyclopentyl ether and the other meta-phenylenediamine(hardening agent). The composite was cured at C for 2 hours and 150C for4 hours at a pressure of approximately psi. As indicated in the resultstabulated below in table 1, there was a small loss in the tensilestrength, but there was a tremendous improvement in the shear strength.The shear test results are an average of fourdifferent tests made oneach sample.

TABLE 11 Physical Properties of 0 Composites The data in the foregoingtable show that very high shear strength values were obtained whengraphite yarn treated according to the process of this invent-ion atvarious current densities was used to reinforce the resin.

'lAllLE 1 Fiber properties Composite properties (-nrrent '1 nsile WeightFiber S p.s.i., density, strength, change, Sp. gr. vohnne, '1hick.,Width, L/dzli/l, L/dz4/1, :m1p./en1.'- k.p.s.i. percent g./eni. percentin. in. max. max.

418 0. 0 1. 415 48 0. 077 0. 080 4, 300 5, 500 356 7. 4 1. 461 58 0. 0790. 077 7, 800 10, 800 308 3. 7 1. 442 55 0.070 0.077 7, 700 9, 100 1Speeifie gravity. 2 Length/depth. 3 Short beam shear strength.

EXAMPLE 1 EXAMPLE 111 A series of comparative tests was made, followingthe 45 A series of experiments was conducted using the procedureprocedure of example 1 and using a number of strands of graphite yarnall from a different bobbin from that used in example 1. In preparingthe resin composite, the proportion was adjusted to 60 volume percentfiber. The axes of the fibers in the of example 1 and varying thecurrent density. The properties of the treated fibers and theresin-fiber composites are reported in table 111. In each run the dwellor residence of the yarn in the electrolyte bath was 150 seconds.

TABLE III [Short beam shear strength of 0 composite] Fiber propertiesComposite properties Tensile Current Weight Fiber Sn, 1 .s.i.,

density, Strength, Mod., change, Sp. gin, volume, Thick, Width, L/dz4/l,

llllllL/CllL k.p.s.i. M.p.s.i. percent g./crn. percent in. in. max.

285 38. 4 0 1. 430 51 0. 070 (l. 113 6, 200 274 38. 0 1. 0 1. 430 51 0.0711 0. 118 (i, 800 314 45. 1 -5 1. 410 45 0. 079 0. 7, 000 228 40. 6 1.5 1. 300 41 0. 079 0. 121 8, 200 234 41. 9 0 l. 400 44 0. 079 0. 110 8,800 252 43. 7 (i 1. 38 i 40 0. 080 0. 101 10, 300 300 41. 7 -0. 2 1. 40045 0. 080 0. 113 13, 300 266 39. 8 2 1. 386 40 0. 081 0. 116 12, J00 20837. 4 5 1. 16 47 0.078 0. J, 600 309 41. ti 0 1. 411 46 0. 079 0. 11.40, 600 303 41. 4 0 1. 405 45 0. 080 0. 113 6, 1100 298 40. 0 0 1. 401 440. 079 0. 113 7, 100

l Specific gravity. 9 Length/depth. 3 Short beam shear strength.

composites were parallel to one another (0 composites). The results aretabulated below in table ll.

The data in the foregoing table demonstrate the improve- 75 mentobtained in composite properties when using the treated fibers of thisinvention as the reinforcing material as compared to the use ofuntreated fibers. The data also show that maximum shear strength valueswere obtained at current densities of about 0.0010 and 0.001 8 amplcmEXAMPLE IV The procedure of example l is repeated a number of timesusing in place of the epoxy resin an equal weight respectively ofphenol-formaldehyde resin, a polyimide resin, nylon, polyurethane,polyethylene and polystyrene. In each case improvement in shear strengthoccurs as compared to use of untreated fibers.

EXAMPLE V The procedure of example 1 was repeated with similar resultsusing a 5 percent aqueous phosphoric acid solution instead of the sodiumhydroxide solution. Improvements also occur when aqueous solutions ofnitric acid, boric acid or sulfuric acid are used as the electrolyte.

EXAMPLE Vl Similar improvements occur when a bundle of carbon fibers issuspended in the bath and other conditions applied as in example l withthe anode charge being applied to the bundle of carbon fibers.

In the foregoing examples, the physical property data were obtainedaccording to the following test methods:

1. Tensile strength-in determining tensile strength, the strand of yarnwas impregnated with a liquid matrix resin to hold the plies in place.Any excess resin was wiped off after which the strand was placed in anoven at 150C. for minutes. At least one plastic bead was then threadedonto each end of the strand, The beads were fastened in place on theresin by injecting adhesive inside the hole in the bead. The adhesivewas cured by heating at 100C for 30 minutes. Each end of the strand wasthen placed in a holding device having a Breaking load, lbs.

area, in.

: Number of filaments y avg. filament cross-section 2. Short beam shearstrengthdetermined according to the test method ASTM D2344-67-T.

We claim:

1. A process for improving the bonding properties of a carbon orgraphite fiber to a resin or plastic matrix which comprises the steps ofsubjecting a carbon or graphite fiber to an electrolytic reaction for aperiod in the range of about 25 to 500 seconds whereby negative ions ofan aqueous electrolyte with which said fiber is in contact are attractedto said fiber acting as an anode and nascent oxygen is produced at thesurface of said fiber, thereby modifying the fiber surface; andcontrolling current density used in the electrolytic reaction so that itis in the range of about 0.0005 to 0.005 ampere per square centimeter ofsurface area of said fiber in contact with said electrolyte.

2. The process according to claim 1 in which said fiber is passedthrough said electrolyte as a continuous strand.

3. The process according to claim 1 in which said current density is inthe range of about 0.001 to 0.003 ampere per square centimeter.

4. The process according to claim 1 in which said electrolyte is anaqueous solution of a compound selected from the group consisting ofsodium hydroxide, potassium hydroxide, phosphoric acid, nitric acid,boric acid and sulfuric acid and the concentration of said compound insaid solution is in the range of about 0.5 to 20 weight percent.

5. A process for improving the bonding properties of a carbon orgraphite fiber to a resin or plastic matrix which comprises passing acontinuous strand of said fiber through a bath containing an aqueouselectrolyte, said electrolyte having an electrode immersed therein andconnected to a negative pole of a direct current source and said strandprior to entry into said bath passing over a graphite roller connectedto a positive pole of said direct current source; controlling the speedat which said strand is passed through the electrolyte so that it is incontact therewith and subjected to an electrolytic reaction for a periodin the range of about 25 to 500 seconds whereby negative ions of theelectrolyte are attracted to said fiber and nascent oxygen is producedat its surface, thereby modifying the fiber surface; and controllingcurrent density used in the electrolytic reaction so that it is in therange of about 0.0005 to 0.005 ampere per square centimeter of surfacearea of the fiber in contact with said electrolyte.

2. The process accordiNg to claim 1 in which said fiber is passedthrough said electrolyte as a continuous strand.
 3. The processaccording to claim 1 in which said current density is in the range ofabout 0.001 to 0.003 ampere per square centimeter.
 4. The processaccording to claim 1 in which said electrolyte is an aqueous solution ofa compound selected from the group consisting of sodium hydroxide,potassium hydroxide, phosphoric acid, nitric acid, boric acid andsulfuric acid and the concentration of said compound in said solution isin the range of about 0.5 to 20 weight percent.
 5. A process forimproving the bonding properties of a carbon or graphite fiber to aresin or plastic matrix which comprises passing a continuous strand ofsaid fiber through a bath containing an aqueous electrolyte, saidelectrolyte having an electrode immersed therein and connected to anegative pole of a direct current source and said strand prior to entryinto said bath passing over a graphite roller connected to a positivepole of said direct current source; controlling the speed at which saidstrand is passed through the electrolyte so that it is in contacttherewith and subjected to an electrolytic reaction for a period in therange of about 25 to 500 seconds whereby negative ions of theelectrolyte are attracted to said fiber and nascent oxygen is producedat its surface, thereby modifying the fiber surface; and controllingcurrent density used in the electrolytic reaction so that it is in therange of about 0.0005 to 0.005 ampere per square centimeter of surfacearea of the fiber in contact with said electrolyte.